Liquid crystal devices are used for display purposes and for a variety of other purposes utilizing the ability of liquid crystals to switch between two light-modulating characteristics. When an electrical potential is applied to a liquid crystal layer, the liquid crystals of such layer are polarized such that the layer becomes, for example, light-transmissive.
Retention of light-tranmissivity may be described as the time over which an LC-layer (liquid crystal layer) can pass light therethrough after cessation of the application of electrical potential or other excitation. Such retention is typically of very short duration in LC-layers, including those of the type formed of polymeric material holding micro-volumes of liquid crystal material. Indeed, quick reorientation to the non-field condition if often desirable.
A number of patents disclose information concerning retention of polarization. These include:
U.S Pat. No. 4,221,471 (Gurtler), which relates to a liquid crystal memory device and states that memory-stored information (and presumably the related image) is erased merely by removing the writing means, indicating that there is no retention of light-transmissiveness; PA1 U.S. Pat. No. 4,693,557 (Fergason), involving a projector employing short-duration images said to last for several seconds; PA1 U.S. Pat. No. 4,814,760 (Johnston et al.), involving a device said to need refreshing at a high frequency in order to permit the image to be seen without interruption; PA1 U.S. Pat. No. 4,433,900 (Sekimura), which presents something of a different situation in that once formed, by extremely long applications of voltage, polarization is fixed; and PA1 U.S. Pat. No. 4,671,618 (Wu et al.), involving a so-called polymer-dispersed liquid crystal material (a form of polymeric material holding micro-volumes of liquid crystal, as in this invention) said to have a memory time on the order of a few seconds or longer, such limited times being referred to as "extended memory."
The Wu et al. patent indicates that to achieve such "extended times" the LC-layer should contain at least 50% liquid crystal material.
It is apparent that it is desirable in certain liquid crystal applications to have polarization retention in a switchable system for a longer period than contemplated in such patents. For a writing/drawing slate or the like, a period of a few seconds is often inadequate.
The prior art includes certain liquid crystal devices which might be referred to as slates--devices addressed or imaged using stylus-like devices. Some examples of this are disclosed in U.S. Pat. No. 3,781,085 (Liebowitz), U.S. Pat. No. 4,525,032 (Hilsum), U.S. Pat. No. 4,723,836 (Kono et al.), and U.S. Pat. No. 3,886,311 (Rodgers et al.).
Such devices have certain disadvantages, particularly the relatively short duration of the image following cessation of the application of electrical potential. This disadvantage heretofore has made liquid crystal devices generally unsuitable as writing/display slates and for certain other applications.
The teachings in the Wu et al. patent about duration of polarization not only fail to provide what is needed for effective writing/drawing slates and the like, but the Wu et al. teaching of the use of large amounts of liquid crystal in the polymer-dispersed liquid crystal material tends to make any resulting product expensive because of the high cost of liquid crystal material.
A liquid crystal device having extended duration of the light-modulating characteristic induced by application of potential to an LC-layer would be a significant advance in the art.